The present invention relates to a variable speed control switch for an electric tool including a DC motor, and more particularly relates so such a variable speed control switch for an electric tool including a DC motor, which is light and compact and therefore permits compact and light construction of the electric tool as a whole.
The present inventors wish hereto to attract the attention of the examining authorities to copending patent application Ser. No. 944,221, which may be considered to be material to the examination of the present patent application.
In the prior art, there have been proposed various types of variable speed control switch for an electric tool including a DC motor. According to such a DC motor as for example utilized in such an electric tool which may be a hand held tool, the output torque rapidly increases as the load is increased and the rotational speed of the DC motor drops, and if the DC motor is continued to be operated in such a state there is a danger of an excessive torque output being produced and of the motor being burnt out. Therefore it has been conceived of to incorporate a torque limiter device into such a variable speed control switch for an electric tool including a DC motor, so as to drive the motor at a fixed torque or at a torque not higher than a determinate torque; but, since the torque must be adjusted for each different application and moreover the addition of such a torque limiter device increases the complexity and the bulk of the variable speed control switch and accordingly boosts the cost thereof, such a solution is by no means ideal. Further, prior art type DC motor variable speed control switches in general have tended to be complicated and/or bulky.
Typically, further, a variable speed control switch for an electric tool including a DC motor has included an ON/OFF switch for turning on and off the power source of the DC motor, a variable resistor for adjustably varying the resistance value of a speed varying control circuit, a brake switch for applying braking force to the motor when the motor is stopped by using the electromotive force generated by the inertia of the motor, and various other elements, and accordingly such a variable speed control switch for an electric tool including a DC motor has inevitably tended to be large and bulky and also heavy and complicated.
Further, typically such a variable speed control switch for an electric tool including a DC motor has been installed into the grip portion of said electric tool, and, by an operation lever of said electric switch which protrudes from the front surface of said grip portion of said electric tool being depressed by being squeezed by the hand of a user, the contacts of the switch mechanism are switched over so that the DC motor of said electric tool is switched ON and OFF and further the operating speed of said DC motor is varied. However, since the motion of such a conventional type of operating lever is a sliding motion which controls the internal mechanism of the switch by way of a back and forth motion, the effective stroke is no more than the actual depression amount provided by the hand of the user, and accordingly said effective stroke is necessarily rather short. On the other hand, if the effective stroke is made to be so long as is convenient for manufuacturing the switching mechanism, the operation stroke of the switching mechanism is increased so much, and the operation lever protrudes so much from the front surface of the grip of the electric tool, that the usability of said electric tool is severly deteriorated.
Further, with such a slide type operating lever, the problem arises of the entry of dust or dirt or foreign matter into the interior of the switching mechanism. Since the electric tool is typically used for working by processing various materials by grinding them, drilling them, or cutting them, large quantities of such foreign matter such as cuttings, chips, or swarf or the like are nearly inevitably produced in the neighborhood of said switch mechanism of said electric tool, and further since such foreign matter is likely to be of metallic type, the entry thereof into the interior of said switch mechanism can give rise to troublesome problems and in the worst case can totally ruin the operation of said switching mechanism. Accordingly, there is a requirement for extremely effective sealing of such a switching mechanism against external particles or dirt or dust or moisture or the like.
Further, such a variable speed control switch for an electric tool including a DC motor typically includes a variable resistor for the purpose of speed control of said DC motor. However, in a conventional such variable speed control switch, since such a variable resistor is typically provided separately with the electric tool in association with the adjustment and operation unit, not only is the interior space within the electric tool utilized in an inefficient manner, but also the wiring between the variable resistor and a typical printed circuit board within the electric tool tends to become complicated, and as a result this in turn causes inefficiency in the assembly work for the electric tool and attendant increase in production and manufacturing cost.
Yet further, a switching element (such as a power transistor) incorporated in such a variable speed control switch for an electric tool including a DC motor typically tends to generate substantial heat due to its basic structure, and therefore various measures have conventionally been taken for controlling the heat generated by such a switching element such as a power transistor. However, the effectiveness of heat control has not always been optimum, and there have always heretofore been the problems of impaired performance and reduced service life of the switching mechanism due to deterioration thereof caused by the evolved heat.
Yet further, typically such a power transistor to be associated with such a variable speed control switch for an electric tool including a DC motor has its terminals connected to such a printed circuit board, and then subsequently the printed circuit board is connected to the terminals of the switching mechanism, thus connecting the power transistor to the switching mechanism via the printed circuit board.
However, since a relatively large electrical current typically flows through the power transistor and therefore through the printed cicuit board, the output end of the pattern printed on said printed circuit board is inevitably required to be relatively large and thick, and the size of the printed circuit board must undesirably be increased accordingly. And, furthermore, electrically connecting the terminals of such a power transistor or the like and also the terminals of the switching mechanism to the printed circuit board tends to be cumbersome, and the efficiency with which the work can be performed tends to be low, because of the requirement for making connections within the switch casing. Even yet furthermore, these circumstances may contribute to insufficient mechanical strength of such connections, and may cause said connections to fail when subjected to vibration or the like as is inevitable during the use of such an electric tool.